A fuel cell converts chemical energy directly into electrical energy by providing a fuel and an oxidant for two electrically-connected electrodes, and causing electrochemical oxidation of fuel. Unlike thermal power, the fuel cell shows high energy conversion efficiency since it is not subject to the restriction of Carnot cycle. The fuel cell generally has a structure provided with plurality of stacked single cells, each having a fundamental structure of the membrane-electrode assembly in which the electrolyte membrane is interposed between a pair of electrodes. In particular, a solid polymer electrolyte fuel cell using the solid polymer electrolyte membrane as the electrolyte membrane has advantages in easiness to downsize and workability at low temperature or the like, and attention is hence attracted particularly to an employment of the solid polymer electrolyte fuel cell as portable and mobile power supply.
In the solid polymer electrolyte fuel cell, a reaction of formula (1) proceeds at an anode (fuel electrode).H2→2H++2e−  (1)
Electrons generated in the formula (1) reach a cathode (oxidant electrode) after passing through an external circuit and working at an outside load. Then, protons generated in the formula (1) in a state of hydration with water move the inside of the solid polymer electrolyte membrane from its anode side to its cathode side by electro-osmosis.
On the other hand, a reaction of formula (2) proceeds at the cathode.4H++O2+4e−→2H2O  (2)
As mentioned above, since some water molecule accompany protons generated at the anode when the protons transfer to the cathode through the solid polymer electrolyte membrane, the solid polymer electrolyte membrane needs to retain high wet state. Thus, a membrane-electrode assembly may be humidified by providing moisture to reaction gas (fuel gas, oxidant gas).
The wet state of the solid polymer electrolyte membrane in the fuel cell varies by operating status and operating condition of the fuel cell or the like. For example, the wet state of the electrolyte membrane varies by operation status of the fuel cell which is in operation or not. And also it varies by current density, cell temperature and humidified temperature of the reaction gas or the like under the operation environment the fuel cell.
With a variation in the wet state (wet, dry), the electrolyte membrane expands (when wet) and contracts (when dry). The electrolyte membrane which has once expanded or contracted may not return to the original flat condition and crinkle may be formed. Parts of crinkle formed on the electrolyte membrane which is fixed in the membrane-electrode assembly are easily collect water, thereby break of the electrolyte membrane and peeling between the electrolyte membrane and a catalyst layer which is adjacent to the electrolyte membrane or the like are caused. Further, cracks are generated by concentrating deformational stress on the crinkle by repeated expansion and contraction, and then the electrolyte membrane may be eventually broken.
As mentioned above, the crinkle of the electrolyte membrane generated by dimensional change causes a deterioration of the electrolyte membrane and a decline in electric performance of the fuel cell. In addition, the electrolyte membrane on which cracks and breaks are generated causes so-called cross leak which is that the reaction gas passes in a molecular state without ionizing and causes further deterioration of the membrane and other constructional element of the fuel cell.
In order to solve the above problems, various arts are proposed. For example, Patent Document 1 discloses a method of producing an electrolyte membrane having small dimensional change to plane direction in a heating and moisture state of the electrolyte membrane for solid polymer fuel cell and an electrolyte membrane obtained by the method. According to Patent Document 1, it says that it is possible to obtain the electrolyte membrane for the solid polymer fuel cell whose contraction percentage at 160° C. is within a range of 1 to 35%, and whose percentage of dimensional change at 80° C. in wet condition is within a range of −10 to 30% in proportion to the contracting percentage at 23° C. in 50% of the humidity by the method disclosed in the Patent Document 1.
Patent Document 1: Japanese Patent Application Laid-open No. 2005-166329